Hard disk drives (HDD) are used in almost all computer system operations. In fact, most computing systems are not operational without some type of hard disk drive to store the most basic computing information such as the boot operation, the operating system, the applications, and the like. In general, the hard disk drive is a device which may or may not be removable, but without which the computing system will generally not operate.
The basic hard disk drive model includes a storage disk or hard disk that spins at a designed rotational speed. An actuator arm with a suspended slider is utilized to reach out over the disk. The slider is coupled with a suspension that supports both the body of the slider and a head assembly that has a magnetic read/write transducer or head or heads for reading/writing information to or from a location on the disk. The complete head assembly, e.g., the suspension, slider, and head, is called a head gimbal assembly (HGA).
A typical HDD uses the actuator assembly to move magnetic read/write heads to the desired location on the rotating disk so as to write information to or read from the location. Within most HDDs, the magnetic read/write head is mounted on a slider. The slider generally serves to mechanically support the head and any electrical connections between the head and the rest of the disk drive. The slider is aerodynamically shaped to establish an air lubrication film in order to maintain a uniform distance from the surface of the rotating disk, thereby preventing the head from undesirably contacting the disk.
The head and arm assembly is linearly or pivotally moved utilizing a magnet/coil structure that is often called a voice coil motor (VCM). The stator of a VCM is mounted to a base plate or casting on which the spindle is also mounted. The base casting with its spindle, actuator VCM, and internal filtration system is then enclosed with a cover and seal assembly to ensure that no contaminants can enter and adversely affect the reliability of the slider flying over the disk. When current is fed to the motor, the VCM develops force or torque that is substantially proportional to the applied current. The arm acceleration is substantially proportional to the magnitude of the current. As the read/write head approaches a desired track, a reverse polarity signal is applied to the actuator, causing the signal to act as a brake, and ideally causing the read/write head to stop and settle directly over the desired track.
One of the major hard disk drive (HDD) challenges is track misregistration (TMR). TMR is the term used for measuring off track errors while a HDD writes data to and reads data from the disks. One of the major contributors to TMR is flow-induced vibration. Flow-induced vibration is caused by turbulent flow within the HDD. The nature of the flow inside a HDD is characterized by Reynolds number, which is defined as the product of a characteristic speed in the drive (such as the speed at the outer diameter of the disk), and a characteristic dimension (such as the disk diameter or, for some purposes, disk spacing), divided by the kinematic viscosity of the air. In general, the higher the Reynolds number, the greater the propensity of the flow to be turbulent.
Due to the high rotational speed of the disks and the complex geometry of the HDD components, the flow pattern inside a HDD is inherently unstable and non-uniform in space and time. The combination of flow fluctuations and component vibrations are commonly referred to as flow induced vibration fluid structure interaction, and “flutter.”
A significant portion of flow induced vibration of a HDD with two or more disks is associated with the turbulent flow near the disk outer edge. Every disk surface in a HDD causes radial outflow of the air near its surface. In today's drives the thickness of this layer is typically a few tenths of millimeters near the rim of the disk. The thickness of the layer (called Ekman layer) is much smaller than the disk to disk axial clearance which is typically of the order of 1.5 to 3 mm. At the rim of the disk the Ekman layer continues in the form of a jet flow directed at the disk cylindrical enclosure (called a shroud). Near the disk/shroud region the jets on either side of the disk bend sharply over an angle of 180 degrees and dissipate. Between two disk surfaces the two jets flow together originating from a line on the disk shroud called a rear stagnation line. The jets also entrain and mix with air adjacent to the shroud.
In a majority of disk drives built today, the rear stagnation line is not stable but wobbles chaotically about the mean axial location of two adjacent co-rotating disks. The circumferential velocity of the air in the re-entrant Ekman jets is actually lower than the circumferential speed of the nearby disk surfaces. The unsteady burst-like Ekman jet chaotically injects air with low circumferential velocity which mixes with higher velocity air. The mixing process causes a Reynolds stress which tends to slow down the disks.
The unsteadiness of the Ekman jet is undesirable. It is therefore desirable to have a shroud that assures that the Ekman jet always leaves the shroud at a fixed location thereby minimizing the amount of turbulence in the HDD.